Walking and balance tests are used for evaluating the effects of rehabilitation and the time taken to perform the test. The Four Square Step Test (FSST) is a useful test for evaluating balance in patients with stroke and orthopedic diseases. For the FSST, 4 bars are used to divide a floor surface into 4 areas. Subjects have to step from one area into the neighboring area in a sequence, and their movements and stability are evaluated. However, no detailed evaluation has been conducted on the transition of the forward, backward, right, and left movement phases. This study aimed at detailed evaluation of the FSST in post-stroke hemiplegic patients and elderly subjects, using wearable motion sensors comprising an accelerometer and angular velocity sensors capable of identifying the direction of the subjects' movements. Twelve post-stroke hemiplegic patients (6 left hemiplegic patients, 6 right hemiplegic patients; Brunnstrom stage of the lower limb: V) and 6 normal elderly subjects were studied. The sensor was attached to the waist and both thighs of a subject. Then FSST was administered and a video was recorded simultaneously. Using the angular velocity signals of the thigh, the four movement phases in FSST were classified. The following items were analyzed: root mean square (RMS) acceleration, RMS angular velocity, amplitude of the thigh angle, and amplitude/performing time. The time measured by the wearable motion sensors correlated with the time recorded by the monitoring video (r = 0.88, p < 0.05). The total performing time did not differ significantly between normal elderly subjects and hemiplegic patients. The RMS acceleration and RMS angular velocity for the waist were significantly different between normal elderly subjects and hemiplegic patients (p < 0.05). Furthermore, the changes in the thigh angle of patients when they stepped over the bars while performing the FSST were different between hemiplegic patients and normal elderly subjects. We conclude that the FSST using wearable motion sensors is useful for the evaluation of balance in post-stroke hemiplegic patients.